Control of downy mildew of grapevines by boosting their natural defence system
Abstract
Fourteen potential elicitors of natural disease resistance were screened for activity against downy mildew on grapevines. Three of these products, Brotomax, AlCl3 and chitosan had high levels of efficacy against downy mildew. It was not proven conclusively that these three products activate host defence responses. In contrast to most inducers of plant defence, Brotomax, AlCl3 and chitosan all had direct antifungal activity against zoospores of the pathogen. Brotomax and AlCl3 also appeared to have some systemic activity while chitosan did not. Contact fungicide activity appeared to be responsible for most of the disease control observed. Both Brotomax and chitosan treatments were trialed in the field where they provided equivalent protection to the standard protectant fungicide (copper oxychloride) on leaves but had lower efficacy than the standard treatments on developing bunches. All products tested showed no post-infection activity and must therefore be used before infections become established.
Brotomax is currently marketed in Australia as a plant nutrient for grapevines. However, at the dose rates required for downy mildew control, it would be approximately six times more expensive than current synthetic fungicides. This high cost would be a major barrier to the adoption of Brotomax as a fungicide product. Chitosan is a nontoxic and biodegradable product produced from crustacean waste. While chitosancontaining crop protection products are available in Australia, they are not registered as fungicides and at current prices, would not be economically viable for downy mildew control. An Australian potassium bicarbonate product (Ecocarb) was found to have high levels of activity against downy mildew in the glasshouse in a trial conducted late in the project but further trials were not conducted due to time constraints.
Summary
14 potential elecitors of plant defence were tested 3 of which were effective against PM but mainly as preventive zoospore killers. The required doses for bunch control would not be economical.